Insights on advanced substrates for controllable fabrication of photoanodes toward efficient and stable photoelectrochemical water splitting

Conversion of solar energy into H_(2) by photoelectrochemical(PEC)water splitting is recognized as an ideal way to address the growing energy crisis and environmental issues.In a typical PEC cell,the construction of photoanodes is crucial to guarantee the high efficiency and stability of PEC reactions,which fundamentally rely on rationally designed semiconductors(as the active materials)and substrates(as the current collectors).In this review work,we start with a brief introduction of the roles of substrates in the PEC process.Then,we provide a systematic overview of representative strategies for the controlled fabrication of photoanodes on rationally designed substrates,including conductive glass,metal,sapphire,silicon,silicon carbide,and flexible substrates.Finally,some prospects concerning the challenges and research directions in this area are proposed.

Direct photoelectrochemical nitrate synthesis in an acidic electrolyte from N_(2)by a hole coupled oxygen atom transfer process

Ammonia is an important chemical for pharmaceutical,agriculture,industry,as well as energy production et al.However,the industrial production of ammonia using the Haber-Bosch process is energy-intensive,which stimulates us to explore a cost-effective and low-carbon footprint way for the synthesis of ammonia[1–3].Electrochemical(EC)synthesis of ammonia from an aqueous N_(2)reduction reaction(NRR)has gained significant attention in recent years,while the high dissociation energy of the N≡N bond(941 kJ/mol),as well as higher over-potential than hydrogen evolution reaction(HER),cause a lower efficiency[4].

Significantly enhanced photoelectrochemical cathodic protection performance and preventing biofouling dual functional Cu_(2)ZnSnS_(4)/TiO_(2)nanotube composite material

A new type of photoelectrochemical cathodic protection technology(a combination of seawater corrosion and biological fouling resistance)is being actively researched to alleviate the serious corrosion of marine metal materials.At present,there is almost no research on anti-corrosion and anti-fouling dual functional materials.In this paper,Cu_(2)ZnSnS_(4)is attached to the surface of TiO,nanotubes through a one-step hydrothermal method for modification.The results indicate that when the hydrothermal reaction time is 24 h,Cu_(2)ZnSnS_(4)/TiO_(2)nanocomposite material exhibits excellent performance in coupling with the protected 304 SS,with its open circuit potential shifts negatively to-1.04 V.This material improves the separation efficiency of photogenerated electrons and effectively improves the photochemical cathodic protection of 304 stainless steel.The high removal rate of Staphylococcus aureus(up to 93%)of the as-prepared samples also proved that it has the effect of the anti-biological fouling.

Rate Law for Photoelectrochemical Water Splitting Over CuO

Photocatalytic splitting of water over p-type semiconductors is a promising strategy for production of hydrogen.However,the determination of rate law is rarely reported.To this purpose,copper oxide(CuO)is selected as a model photocathode in this study,and the photogenerated surface charge density,interfacial charge transfer rate constant and their relation to the water reduction rate(in terms of photocurrent)were investigated by a combination of(photo)electrochemical techniques.The results showed that the charge transfer rate constant is exponential-dependent on the surface charge density,and that the photocurrent equals to the product of the charge transfer rate constant and surface charge density.The reaction is first-order in terms of surface charge density.Such an unconventional rate law contrasts with the reports in literature.The charge density-dependent rate constant results from the Fermi level pinning(i.e.,Galvani potential is the main driving force for the reaction)due to accumulation of charge in the surface states and/or Frumkin behavior(i.e.,chemical potential is the main driving force).This study,therefore,may be helpful for further investigation on the mechanism of hydrogen evolution over a CuO photocathode and for designing more efficient CuO-based photocatalysts.

High-performance self-driven broadband photoelectrochemical photodetector based on reduced graphene oxide/Bi_(2)Te_(3)heterojunction

Attributed to its excellent physicochemical properties,graphene(GR)has very active applications in the fields of catalysis,optoelectronic devices,and battery electrode materials.However,until now,regulating the type and density of carriers in GR is still crucial for its practical applications.Here,reduced graphene oxide(RGO)-Bi_(2)Te_(3)heterojunctions doped with different contents were prepared by a simple one-step method.The Bi_(2)Te_(3)materials containing different RGO were made into broadband(365–850 nm)photoelectrochemical-type detectors,and the effects of the doping amount of RGO on the optoelectronic behavior of the devices and the intrinsic operation mechanism of the devices were investigated in detail.The results show that the values of Iph/Idark,Ri,and D*of Bi_(2)Te_(3)/RGO heterojunction devices obtained with 1 mg of RGO doping are 412,6.072 mA/W,and 2.4061010 Jones,respectively.It is anticipated that this work will provide a research basis for future quantitative tuning of the performance of micro-nano devices by GR.

Electrodeposition of Cu_2O/g-C_3N_4 heterojunction film on an FTO substrate for enhancing visible light photoelectrochemical water splitting

An immobilized Cu2O/g-C3N4 heterojunction film was successfully made on an FTO substrate by electrophoretic deposition of g-C3N4 on a Cu2O thin film.The photoelectrochemical（PEC） performance for water splitting by the Cu2O/g-C3N4 film was better than pure g-C3N4 and pure Cu2O film.Under-0.4 V external bias and visible light irradiation,the photocurrent density and PEC hydrogen evolution efficiency of the optimized Cu2O/g-C3N4 film was-1.38 mA/cm^2 and 0.48 mL h^-1 cm^-2,respectively.The enhanced PEC performance of Cu2O/g-C3N4 was attributed to the synergistic effect of light coupling and a matching energy band structure between g-C3N4 and Cu2O as well as the external bias.

Photoelectrochemical degradation of acetaminophen and valacyclovir using nanoporous titanium dioxide

Electrochemically treated nanoporous TiO2 was employed as a novel electrode to assist in the pho- toelectrochemical degradation of acetaminophen and valacyclovir. The prepared electrode was characterized by scanning electron microscopy （SEM） and energy dispersive spectroscopy （EDS）. Cyclic voltammetry （CV）, Mott-Schottky plots, ultraviolet-visible light （UV-vis） absorbance spec- troscopy, and a total organic carbon （TOC） analyzer were employed to investigate the photoelec- trochemical degradation of acetaminophen and valacyclovir. The results indicated no obvious re- moval of acetaminophen and valacyclovir over 3 h when separate photochemical degradation and electrochemical oxidation were employed. In contrast, acetaminophen and valacyclovir were rapid- ly eliminated via photoelectrochemical degradation. In addition, electrochemically treated nanopo~ rous TiO2 electrodes significantly enhanced the efficacy of the photoelectrochemical degradation of acetaminophen and valacyclovir, by 86.96% and 53.12%, respectively, when compared with un- treated nanoporous TiO2 electrodes. This enhanced performance may have been attributed to the formation of Ti3~, Ti2~, and oxygen vacancies, as well as an improvement in conductivity during the electrochemical reduction process. The effect of temperature was further investigated, where the activation energy of the photoelectrochemical degradation of acetaminophen and valacyclovir was determined to be 9.62 and 18.42 kJ/mol, respectively.

Advances in Z-scheme semiconductor photocatalysts for the photoelectrochemical applications: A review

With continuous consumption of nonrenewable energy,solar energy has been predicted to play an essential role in meeting the energy demands and miti gating environmental issues in the future.Despite being green,clean and pollution-free energy,solar energy cannot be adopted directly as it cannot provide sufficiently high energy density to work in the absence of machinery.Thus,it is necessary to develop an effective strategy to convert and store solar energy into chemical energy to achieve social sustainable development using solar energy as the main power source.Photocatalysis,in which semi conductor photocatalysts play a key role,is one of the most promising can didates for realising the effective utilisation of sunlight in a green,low-cost and environmentally friendly method.The photocatalytic efficiency of photo catalysts is considerably influenced by their compositions.Among the various heterostructures,Z-scheme heterojunction is one of the most interesting ar chitecture due to its outstanding performance and excellent artificial imitation of photosynthesis.Z-scheme photocatalysts have attracted considerable at tention in the past few decades.Herein,we review contemporary Z-scheme systems,with a particular focus on mechanistic breakthroughs,and highlight current state-of-the-art systems.Z-type photocatalysts are classified as tradi tional,all-solid-state,direct Z-schemes and S-scheme photocatalysts.The morphology,characterisation and working mechanism of each type of Z-scheme are discussed in detail.Furthermore,the applications of Z-scheme in photoelectrochemical water splitting,nitrogen fixation,pollutant degrada tion and carbon dioxide reduction are illustrated.Finally,we outline the main challenges and potential advances in Z-scheme architectures,as well as their future development directions.

Characterization and photoelectrochemical performance of Zn-doped TiO_2 films by sol-gel method

Zn-doped TiO2 (Zn?TiO2) thin films were prepared by the sol?gel method on titanium substrates with heat treatment at different temperatures. The effects of heat treatment temperatures and Zn doping on the structure, photocathodic protection and photoelectrochemical properties of TiO2 thin films were investigated. It is indicated that the photoelectrical performance of the Zn?TiO2 films is enhanced with the addition of Zn element compared with the pure-TiO2 film and the largest decline by 897 mV in the electrode potential is achieved under 300 °C heat treatment. SEM?EDS analyses show that Zn element is unevenly distributed in Zn?TiO2 films; XRD patterns reveal that the grain size of Zn?TiO2 is smaller than that of pure-TiO2; FTIR results indicate that Zn - O bond forms on Zn?TiO2 surface. Ultraviolet visible absorption spectra prove that Zn?TiO2 shifts to visible light region.Mott?Shottky curves show that the flat-band potential of Zn?TiO2 is more negative and charge carrier density is bigger than that ofpure-TiO2, implying that under the synergy of the width of the space-charge layer, carrier density and flat-band potential, Zn?TiO2 with 300 °C heat treatment displays the best photocathodic protection performance.

A review on photoelectrochemical cathodic protection semiconductor thin films for metals

Photoelectrochemical(PEC) cathodic protection is considered as an environment friendly method for metals anticorrosion. In this technology, a n-type semiconductor photoanode provides the photogenerated electrons for metal to achieve cathodic protection. Comparing with traditional PEC photoanode for water splitting, it requires the photoanode providing a suitable cathodic potential for the metal, instead of pursuit ultimate photon to electric conversion efficiency, thus it is a more possible PEC technology for engineering application. To date, great efforts have been devoted to developing novel n-type semiconductors and advanced modification method to improve the performance on PEC cathodic protection metals. Herein, recent progresses in this field are summarized. We highlight the fabrication process of PEC cathodic protection thin film, various nanostructure controlling, doping, compositing methods and their operation mechanism. Finally, the current challenges and future potential works on improving the PEC cathodic protection performance are discussed.

Recent advances in non-metal doped titania for solar-driven photocatalytic/photoelectrochemical water-splitting

Photocatalytic (PC) / Photoelectrochemical (PEC) water splitting under solar light irradiation is considered as a prospective technique to support the sustainable and renewable H_(2) economy and to reach the ultime goal of carbon neutral. TiO_(2) based photocatalysts with high chemical stability and excellent photocatalytic properties have great potential for solar-to-H_(2) conversion. To conquer the challenges of the large band-gap and rapid recombination of photo generated electron-holepairs in TiO_(2), non-metal doping turns out to be economic, facile, and effective on boosting the visible light activity. The localized defect states such as oxygen vacancy and Ti^(3+) generated by non-metal doping are located in the band-gap of TiO_(2), which result in the reduction of band-gap, thus a red-shift of the absorption edge. The hetero doping atoms such as B^(3+), I^(7+), S^(4+)/S^(6+), P^(5+) can also act as electron donors or trap sites which facilitate the charge carrier separation and suppress the recombination of electron-hole pairs. In this comprehensive review, we present the most recent advances on non-metal doped TiO_(2) photocatalysts in terms of fundamental aspects, origin of visible light activity and the PC / PEC behaviours for water splitting. In particular, the characteristics of different non-metal elements (N, C, B, S, P, Halogens) as dopants are discussed in details focusing on the synthesis approaches, characterization as well as the efficiency of PC and PEC water splitting. The present review aims at guiding the readers who want quick access to helpful information about how to efficiently improve the performance of photocatalysts by simple doping strategies and could stimulate new intuitive into the new doping strategies.

Enhanced Performance of a Monolayer MoS_2/WSe_2 Heterojunction as a Photoelectrochemical Cathode

Transition-metal dichalcogenide(TMD) semiconductors have attracted interest as photoelectrochemical(PEC) electrodes due to their novel band-gap structures,optoelectronic properties, and photocatalytic activities.However, the photo-harvesting efficiency still requires improvement. In this study, A TMD stacked heterojunction structure was adopted to further enhance the performance of the PEC cathode. A P-type WSe_2 and an N-type Mo S_2 monolayer were stacked layer-by-layer to build a ultrathin vertical heterojunction using a micro-fabrication method.In situ measurement was employed to characterize the intrinsic PEC performance on a single-sheet heterostructure.Benefitting from its built-in electric field and type II band alignment, the MoS_2/WSe_2 bilayer heterojunction exhibited exceptional photocatalytic activity and a high incident photo-to-current conversion efficiency(IPCE). Comparing with the monolayer WSe_2 cathode, the PEC current and the IPCE of the bilayer heterojunction increased by a factor of 5.6 and enhanced 50%, respectively. The intriguing performance renders the MoS_2/WSe_2 heterojunction attractive for application in high-performance PEC water splitting.

Recent Advances in Visible-Light-Driven Photoelectrochemical Water Splitting: Catalyst Nanostructures and Reaction Systems

Photoelectrochemical(PEC) water splitting using solar energy has attracted great attention for generation of renewable hydrogen with less carbon footprint, while there are enormous challenges that still remain for improving solar energy water splitting efficiency, due to limited light harvesting, energy loss associated to fast recombination of photogenerated charge carriers, as well as electrode degradation. This overview focuses on the recent development about catalyst nanomaterials and nanostructures in different PEC water splitting systems. As photoanode, Au nanoparticle-decorated TiO_2 nanowire electrodes exhibited enhanced photoactivity in both the UV and the visible regions due to surface plasmon resonance of Au and showed the largest photocurrent generation of up to 710 nm. Pt/Cd S/CGSe electrodes were developed as photocathode. With the role of p–n heterojunction, the photoelectrode showed high stability and evolved hydrogen continuously for more than 10 days. Further, in the Z-scheme system(Bi_2S_3/TNA as photoanode and Pt/Si PVC as photocathode at the same time), a self-bias(open-circuit voltage Voc= 0.766 V) was formed between two photoelectrodes, which could facilitate photogenerated charge transfers and enhance the photoelectrochemical performance, and which might provide new hints for PEC water splitting. Meanwhile, the existing problems and prospective solutions have also been reviewed.

Efficient Photoelectrochemical Water Splitting by g-C_3N_4/TiO_2 Nanotube Array Heterostructures

Well-ordered TiO_2 nanotube arrays(TNTAs)decorated with graphitic carbon nitride(g-C_3N_4) were fabricated by anodic oxidization and calcination process.First, TNTAs were prepared via the anodic oxidation of Ti foil in glycerol solution containing fluorinion and 20%deionized water. Subsequently, g-C_3N_4 film was hydrothermally grown on TNTAs via the hydrogen-bonded cyanuric acid melamine supramolecular complex. The results showed that g-C_3N_4 was successfully decorated on the TNTAs and the g-C_3N_4/TNTAs served as an efficient and stable photoanode for photoelectrochemical water splitting. The facile deposition method enables the fabrication of efficient and low-cost photoanodes for renewable energy applications.

Decorating non-noble metal plasmonic Al on a TiO2/Cu2O photoanode to boost performance in photoelectrochemical water splitting

Designing low-cost and high-performance photoelectrodes with improved light harvesting and charge separation rates is significant in photoelectrochemical water splitting.Here,a novel TiO2/Cu2O/Al/Al2O3 photoelectrode is manufactured by depositing plasmonic nanoparticles of the non-noble metal Al on the surface of a TiO2/Cu2O core/shell heterojunction for the first time.The Al nanoparticles,which exhibit a surface plasmon resonance(SPR)effect and are substantially less expensive than noble metals such as Au and Ag,generate hot electron-hole pairs and amplify the electromagnetic field at the interface under illumination.The as-prepared TiO2/Cu2O/Al/Al2O3 photoelectrodes have an extended absorption range and enhanced carrier separation and transfer.Their photocurrent density of 4.52 mA·cm^-2 at 1.23 V vs.RHE represents an 1.84-fold improvement over that of TiO2/Cu2O.Specifically,the ultrathin Al2O3 passivation layer spontaneously generated on the surface of Al in air could act as a protective layer to significantly increase its stability.In this work,the synergistic effect of the heterojunctions and the SPR effect of the non-noble metal Al significantly improve the photoelectrode performance,providing a novel concept for the design of electrodes with good properties and high practicability.

Highly Enhanced Visible-Light-Driven Photoelectrochemical Performance of ZnO-Modified In_2S_3 Nanosheet Arrays by Atomic Layer Deposition

Photoanodes based on In_2S_3/ZnO heterojunction nanosheet arrays(NSAs) have been fabricated by atomic layer deposition of ZnO over In_2S_3 NSAs, which were in situ grown on fluorine-doped tin oxide glasses via a facile solvothermal process. The as-prepared photoanodes show dramatically enhanced performance for photoelectrochemical(PEC) water splitting, compared to single semiconductor counterparts. The optical and PEC properties of In_2S_3/ZnO NSAs have been optimized by modulating the thickness of the Zn O overlayer. After pairing with ZnO, the NSAs exhibit a broadened absorption range and an increased light absorptance over a wide wavelength region of 250–850 nm. The optimized sample of In_2S_3/ZnO-50 NSAs shows a photocurrent density of 1.642 m A cm^(-2)(1.5 V vs. RHE) and an incident photonto-current efficiency of 27.64% at 380 nm(1.23 V vs.RHE), which are 70 and 116 times higher than those of the pristine In_2S_3 NSAs, respectively. A detailed energy band edge analysis reveals the type-II band alignment of the In_2S_3/ZnO heterojunction, which enables efficient separation and collection of photogenerated carriers,especially with the assistance of positive bias potential, and then results in the significantly increased PEC activity.

A molecular cobaloxime cocatalyst and ultrathin FeOOH nanolayers co-modifiedBiVO_(4) photoanode for efficient photoelectrochemical water oxidation

BiVO_(4) has been attracting a lot of interest in photoelectrochemical (PEC) water oxidation due to its efficient solar absorption and appropriate band positions.So far,sluggish water oxidation kinetics and fast photogenerated charge recombination still hinder the PEC performance ofBiVO_(4) .In this study,a novel PEC photoanode was designed by depositing ultrathin FeOOH nanolayers on the surface of nanoporousBiVO_(4) electrode,followed by modification with a cobaloxime (Co(dmgH)_(2)(4-COOH-py)Cl) molecular cocatalyst.Under irradiation of a 100 mW cm^(-2)(AM 1.5G) Xe lamp,the photocurrent density of the cobaloxime/FeOOH/BiVO_(4) composite photoanode reached 5.1 mA cm^(-2)at 1.23 V vs.RHE in 1.0 M potassium borate buffer solution (pH=9.0).The onset potential of the optimal cobaloxime/FeOOH/BiVO_(4) photoanode exhibited a 460 m V cathodic shift relative to bareBiVO_(4) .In addition,the surface charge injection efficiency of the composite photoanode reached~80%at 1.23 V vs.RHE and the incident photon-to-current efficiency (IPCE) reached~88%at 420 nm.

A Facile Self-assembly Synthesis of Hexagonal ZnO Nanosheet Films and Their Photoelectrochemical Properties

Here, large-scale and uniform hexagonal zinc oxide(ZnO) nanosheet films were deposited onto indium tin oxide(ITO)-coated transparent conducting glass substrates via a facile galvanic displacement deposition process. Compared with other commonly used solution methods, this process avoids high temperature and electric power as well as supporting agents to make it simple and cost-effective. The as-fabricated ZnO nanosheet films have uniform hexagonal wurtzite structure. The photoelectrochemical(PEC) cell based on ZnO nanosheet film/ITO photoelectrode was also fabricated and its performance was improved by optimizing the solution concentration. A higher photocurrent density of*500 l A cm^(-2)under AM 1.5 G simulated illumination of 100 m W cm^(-2)with zero bias potential(vs. Ag/AgCl electrode) was obtained, which may ascribe to the increased surface-to-volume ratio of disordered Zn O nanosheet arrays. Our developed method may be used to deposit other oxide semiconductors, and the Zn O nanosheet film/ITO PEC cell can be used to design low-cost optoelectronic and photoelectrochemical devices.

Amorphous TiO_2-modified CuBi_2O_4 Photocathode with enhanced photoelectrochemical hydrogen production activity

In this study,CuBi2O4 photocathodes were prepared using a simple electrodeposition method for photoelectrochemical(PEC)hydrogen production.The prepared photocathodes were modified with amorphous TiO2 and a Pt co‐catalyst,which resulted in the formation of CuBi2O4/TiO2 p‐n heterojunctions,and enhanced the activities of the as‐prepared photocathodes.The novel Pt/TiO2/CuBi2O4 photocathode exhibited a photocurrent of 0.35 mA/cm2 at 0.60 V vs.Reversible Hydrogen Electrode(RHE),which was nearly twice that of the Pt/CuBi2O4 photocathode.The present study provides a facile method for increasing the efficiency of photocathodes and provides meaningful guidance for the preparation of high‐performance CuBi2O4 photocathodes.

Construction of BiVO_(4)/BiOCl@C Z-scheme heterojunction for enhanced photoelectrochemical performance

A Z-scheme heterostructure of Mo,W co-doped BiVO_(4)(Mo,W:BVO/BiOCl@C)was fabricated by a simple solid solution drying and calcination(SSDC)method.The heterostructure was characterized by X-ray diffraction(XRD),Fourier transform infrared(FTIR),X-ray photoelectron spectroscopy(XPS),etc.Under visible light irradiation,Mo,W:BVO/BiOCl@C heterostructure exhibits excellent photoelectrochemical capability compared with other as-prepared samples.The photocurrent density and the incident photon-to-electron conversion efficiency(IPCE)are about 5.4 and 9.0 times higher than those of pure BiVO_(4),respectively.The enhancement of the photoelectrochemical performance can be attributed to the construct of Z-scheme system,which is deduced from the radical trapping experiments.The Mo,W:BVO/BiOCl@C Z-scheme heterojunction enhances the visible-light absorption and reduces the recombination rate of charge carriers.This work provides an effective strategy to construct Z-scheme photoelectrodes for the application of photoelectrochemical water splitting.